Bone density is an important measure of bone health, and in some cases, systemic health of a subject. Low bone density has been identified as a risk factor for fractures (especially long, spinal vertebrae and pelvic bones), degenerative joint disease (arthritis), pain, decreased activity levels, certain disease states (bone cancers, select endocrine diseases, obesity, etc.), medications that result in bone loss, dental disease (due in part to loosened teeth) and even as a measure of welfare. Bone density disorders are recognized in both humans and non-human animals. By identifying poor bone density, clinicians have the opportunity to recognize and diagnose certain diseases earlier (as opposed to waiting for more overt disease to develop) and develop risk assessment protocols and hopefully preventative measures.
In addition, other tissue densities may also show promise for disease identification and serve as prognostic markers of certain diseases. This includes identifying the density of foreign materials that may have an impact on health. For example, by quantifying the density of ingested metals clinicians may be able to determine if conservative therapy results in successful dissolution of the item (by measuring decreasing density over a set period of time). Additionally, non-bone tissues that are more or less radiodense than ‘normal’ may indicate a disease process is present. As an example, hyperadrenocorticism, certain kidney disorders and select toxins can increase mineralization in soft tissues.
Furthermore, low bone density also correlates with poor diet, lack of exercise, and lack of natural light exposure (especially for diurnal species), and may be compared to ‘normals’ to better determine welfare of animals kept in captivity. The ultimate goal would be to improve conditions for captive animals by improving nutrition, activity level and natural UV light exposure, especially for those animals that have restricted access to natural light, sufficient room to ambulate, and/or are on a poor diet. Bone density has been studied in laboratory animals and in poultry species, where low bone density has been found to be a common problem in captive production birds. Advanced cases may easily be recognized by strikingly poor bone density and sometimes folding type fractures on standard radiographs, as shown in the example in FIG. 1. However, studies in other animals are critically lacking primarily due to the cost of diagnostic equipment. As a result, large scale studies that correlate bone/tissue density with health and disease states are not possible without substantial funding.
Advances in medical imaging technology have allowed noninvasive visualization and measurements of a wide variety of anatomy and functions of the body. Radiodensity or radiopacity refers to the relative inability of electromagnetic radiation, particularly X-rays, to pass through a particular material. Radiolucency indicates greater transparency or “transradiancy” to X-ray photons. Materials that inhibit the passage of electromagnetic radiation are called radiodense, while those that allow radiation to pass more freely are referred to as radiolucent. The term refers to the relatively opaque white appearance of dense materials or substances on radiographic imaging studies, compared with the relatively darker appearance of less dense materials. Because calcified tissues such as bone are radio-opaque, X-ray based imaging including projection radiography (or X-ray radiography) and computed tomography (CT) are the most commonly used modalities for assessing bone morphology.
Although X-ray radiography offers the highest spatial resolution useful for detecting, for example, hairline fracture in a bone, due to the lack of calibration and the physics of image formation, X-ray radiography intensities are generally only qualitative in nature. Due to its qualitative nature, X-ray radiographs give clinicians only subjective, relative evaluation of tissue density. As a result, standard radiographs, which are common in private practice, cannot be used to provide scientifically meaningful data on bone/tissue density. In contrast, CT intensities are both quantitative and standardized across all scanners, and are the best (in terms of speed and resolution) for visualizing the skeletal system and some soft tissue structures.
There are several reasons why existing X-ray radiography is not suited for quantitative intensity-based evaluations. Most X-ray radiography and CT instruments employ a “point source” for generating the X-ray. As the generated X-ray radiates away from the source, the intensity of the X-ray decreases as the inverse-square of the distance. Moreover, as the X-ray arrives at the detector, which is normally flat, unless the incident angle is perpendicular to the detector, the intensity of the X-ray is further diminished as the X-ray beam is spread across a bigger area. Combined, even when the point source is aimed directly and squarely at the detector, the “source-detector geometry” imposes an inherent variability on X-ray intensity across the detector. Whether a conventional film or digital detector is used, spontaneous processes in the detector (e.g., intrinsic electronic charges in the digital detector) contributes to baseline intensity in the X-ray image even when the source is completely turned off. Due to the properties of exposure-to-intensity conversion, the conversion might not be linear (i.e., doubling the exposure may not result in doubled brightness on the image). In addition to the baseline and nonlinear responses, all detectors have finite response “dynamic range”. Unless the exposure is optimized to the range, under-exposure can lead to patches of uniformly dense regions (regardless of variability of the underlying anatomy), whereas over-exposure can lead to apparent disappearance of low-density regions.
In computed tomography (CT), all of the above issues with X-ray radiography are effectively addressed by the so-called “dark-light calibration” and “exposure optimization” procedures that are performed as part of the CT acquisition. The dark-light calibration essentially involves obtaining scans with and without the source turned on, and subtracts the obtained values from all subsequent acquisitions. Exposure optimization involves an iterative process of scans and intensity analysis to find the exposure setting that is just below the upper detector dynamic range. Separately, all CT-obtained intensities are standardized by normalizing the intensities to those for air and water, such that air and water will have exactly −1000 and 0 “Hounsfield Units”, respectively, in all scanners.
Even though CT provides the best speed and resolution for visualizing the skeletal system, the cost of CT scans is prohibitive and the limited availability of CT equipment makes its wide usage impractical in most veterinary and human point-of-care practices. Thus, there exists a need for improved systems and methods that provide skeletal visualizations that are comparable to CT scans but at a lower cost and with lower dosimetry.